Key:Trophic Level - The fish's level in the food chain. First level phytoplankton are eaten by the second level zooplankton, which are eaten by the third level anchovies (well slightly less as they also eat some phytoplankton), which are eaten by the fourth level tunas.

NPP/cal - The amount of Net Primary Productivity (phytoplankton) calories per calorie of fish. Each level higher you go on the trophic level, the amount of primary productivity required increases by 10. Second level fish require 10 NPP calories to produce 1 calorie, third level 100 and fourth level 1,000.

Coastal Shelf cal/m2/yr - Amount of calories (kcal) of fish that are generated per square meter of coastal shelf per year. Based on 3,600 calories of NPP per m2.

Open Ocean cal/m2/yr - Amount of calories of fish that are generated per square meter of open ocean per year. Based on 1,000 calories of NPP per m2. Some species, such as scallops, are not found in the open ocean and should just be disregarded.

It takes 100 times as much ocean acreage to produce 1 lb of tuna as it does to produce 1 lb of scallops. You could eat 100 lbs of scallops or 1 lb of tuna and both would require the same amount of net primary productivity. As mentioned in Bottomfeeders, tuna should not be referred to as chicken of the sea but rather wolves. Switching from tuna fish sandwiches to herring fish sandwiches, will cut your footprint 86%. You can eat 7 times as much herring as tuna from the same amount of NPP. Freshwater herbivores like tilapia aren't mentioned here but are also another good choice to eat to minimize your footprint.

Besides reducing your footprint, eating lower on the chain is also healthier. Mercury, heavy metals and other toxins accumulate the higher up the food chain you go. Sardines just made the NY Times 11 Best Foods You Aren’t Eating list.

2) More total tons of fish, and therefore more protein and calories, would be available to humanity if we fished lower on the food chain.

Humans use 8% of total ocean productivity: 2% of open ocean productivity and around 30% of upwelling and shelf systems productivity. This compares with 24% that we use of terrestrial production. While the shelves make up only 9% of total area, they account for 25% of total ocean NPP production. Marine resources provide about 20 percent of the animal protein eaten by humans globally and 5 percent of livestock feed.

There are 332 million km2 of open ocean and 32 million km2 of continental shelves. That works out to 5 ha of open ocean and .5 ha of continental shelves per each of the 6.6 billion human on the Earth.

If humans were to use 10% of ocean productivity, each person could have 6.7 mil calories of NPP. This would allow everyone to consume 18.5 calories a day of trophic level 4 fish (tuna), or 185 calories a day of trophic level 3 (anchovies), or 1,850 calories of trophic level 2 fish (shrimp).

We could eat 10 times as much herring than if we allowed them to be eaten by salmon and then caught and ate the salmon. There would be less salmon to eat, but 10 times more total fish.

Catching more low trophic level fish will give us more food, but will also lower the populations of higher trophic level fish. But, for a given amount of protein/tons of fish, catching from the bottom will minimize the impact on the oceans.

3) High trophic level fish required more acreage per calorie than terrestrial livestock, while lower level ones are fairly similar.

Cattle produce 35 calories per square meter and chickens 368 cal/m2. Tuna on the open ocean produce 1 cal/m2, much lower than either beef or chicken. Scallops from a coastal shelf produce 360 cal/m2 very similar to chicken, and herring on a coastal shelf produce 45.4 cal/m2, slightly more than cattle. Not that ocean acreage and land acreage are interchangeable, but it is interesting to compare them.

4) Trophic level is not reflected in the price of fish.

Lobsters, crabs and scallops are more expensive than higher level fish like cod and salmon. The cost of fish is determined just by how difficult the fish are to catch and bring to market.

If fish were farmed you would see more of a correlation, as higher trophic level fish require more feed to eat and are therefore more expensive to produce. The trophic level of fish would also be reflected in price if property rights to the ocean acreage or the NPP were assigned.

If price better reflected the fish's NPP requirements, people would naturally eat lower on the food chain, and more fish would be available to all.

5) Minimizing trophic level is more important than minimizing bycatch.

Fishing one trophic level higher requires 10 times the amount of NPP. Even if bycatch is 8 times as large as the catch, assuming the bycatch is of the same trophic level as the catch, that would still require less NPP than fishing one trophic level higher.

Caveats and Assumptions:1) This analysis is based on Net Primary Productivity (NPP) rates of 3,600 kcal/m2/yr for coastal shelves and 1,000 kcal/m2/yr for the open ocean. This comes from the gC/m2/yr values in the Nature article then converted to calories (kCal) using a ratio of 1 gC to 10 kCal. The coastal shelf value is an average of all the non-open ocean ecosystems.

Estimates of NPP per m2 vary. This this source (p 497) has similar values of 1,000 kcal/m2 for open ocean, 2,000 for coastal oceans and 6,000 for upwelling zones, but this source was a bit lower at around 500 for open oceans and 1,600 for coastal zones. The Sea Around Us Project also has estimates of primary productivity for different ocean regions around the world.

2) The ratio of 10 to 1 between trophic levels is an approximation and might not hold for all the species listed. One place it has been measured is in farmed salmon, and it was found that it takes between 8.5 to 9.9 kg of pelagic fish in order to produce one kg of farmed salmon in Chile.

On land, both cows and chickens are at the same level of the food chain/trophic level, but cows take much longer to grow and therefore their meat requires more NPP/acres of land to grow corn than a chicken. Likewise, a fast growing fish will require less NPP to get to maturity than one that is slow growing.

This analysis also doesn't take into account what percentage of the fish is edible meat. A low trophic level species like a scallop might have a larger impact than seen as much of their energy goes into creating a non-edible shell.

2 comments:

It is interesting comparing chicken/scallops for land area, but you also bring up a good point that it's not really "land area" in the case of sea food. We can't exactly live on area occupied by the ocean -- except in someone's dreams.